The present invention relates to a PWM (Pulse Width Modulation) control circuit that is comprised of an analog circuit, and is used to match an output voltage from a DCxe2x80x94DC converter with a target value, with the DCxe2x80x94DC converter converting one DC voltage into another DC voltage by turning on and off a semiconductor switch.
FIG. 7 shows a conventional example of a PWM control circuit for a DCxe2x80x94DC converter as described above. In this figure, reference numeral 1 denotes a DCxe2x80x94DC converter for converting one DC voltage into another DC voltage by turning on and off a semiconductor switch and then supplying the resulting DC voltage to a load 3. Reference numeral 4 denotes a PWM control circuit for controlling an output voltage VOUT from the DCxe2x80x94DC converter to a target value, with the PWM circuit being composed of a detection circuit 5, an error amplification circuit 6, a comparison circuit 7, and the like. The detection circuit 5 detects the output voltage VOUT from the DCxe2x80x94DC converter 1 and is composed of, for example, an attenuator, a rectifier, a buffer, an insulated amplifier, or the like. The error amplification circuit 6 amplifies the difference between the output voltage VOxe2x80x2 from the detection circuit 5 and the reference voltage VREF. The comparison circuit 7 compares the output voltage VE from the error amplification circuit 6 with a carrier signal VOSC with a triangular wave or a saw-tooth wave, and outputs a PWM signal.
As described above, the PWM control circuit has the function of feeding back the output voltage from the DCxe2x80x94DC converter and controlling the on/off ratio (duty cycle) of a semiconductor switch, in accordance with the difference from the target value. Specific examples of the DCxe2x80x94DC converter include a buck chopper circuit such as that shown in FIG. 8, a boost chopper circuit such as that shown in FIG. 9, and a buck-boost chopper circuit such as that shown in FIG. 10. In addition, specific examples of the detection circuit 5 and the error amplification circuit 6 are shown in FIG. 11.
In general, the relationship between an input voltage to and an output voltage from an analog amplification circuit is represented by Equation (1):
V2xe2x88x92Vbias=K (V1xe2x88x92Vbias)xe2x80x83xe2x80x83(1)
where V1 and V2 denote an input voltage to and an output voltage from an analog amplification circuit, respectively, Vbias denotes a voltage corresponding to an operation point of the analog amplification circuit, and K denotes a gain of the analog amplification circuit.
The operation point of the conventional error amplification circuit 6 shown in FIG. 11 is the reference voltage VREF, that is, Vbias=VREF. Thus, a problem occurs with the transient response obtained when the VREF is varied. This will be described with reference to FIGS. 12(a) and 12(b).
FIGS. 12(a) and 12(b) show an example in which the DCxe2x80x94DC converter 1 shown in FIG. 7 comprises a voltage-reducing chopper circuit such as that shown in FIG. 8, and in which the detection circuit 5 and the error amplification circuit 6 are configured as shown in FIG. 11. FIG. 12(a) shows an example of an output-voltage waveform from the DCxe2x80x94DC converter obtained when the VREF is varied stepwise as shown in FIG. 12(b).
That is, in a steady state, the output voltage VE from the error amplification circuit 6 has a certain DC value within the amplitude of the carrier signal VOSO in order to generate a PWM signal of a time ratio corresponding to a target value. In this case, when the VREF is varied stepwise, the operation point Vbias of the error amplification circuit 6 simultaneously varies stepwise. Thus, the value of VE also varies stepwise by an amount corresponding to the variation in VREF, as shown by the above Equation (1), and control is then provided so as to achieve a next target value using the resulting VE value as the initial value. Thus, excess voltage may be generated as shown in FIG. 12(a), regardless of the gain of the error amplification circuit 6.
Accordingly, it is an object of the present invention to improve the transition response while preventing the excess of output voltage from the DCxe2x80x94DC converter using the PWM control circuit.
To attain the above object, the first aspect of the present invention provides a PWM control circuit for a DCxe2x80x94DC converter comprising a detection circuit for detecting an output voltage from a DCxe2x80x94DC converter that converts one DC voltage into another DC voltage by turning on and off a semiconductor switch, an error amplification circuit for amplifying a difference between the detected voltage and a reference voltage, and a comparison circuit for comparing an output voltage from the error amplification circuit with a carrier signal with a triangular wave or saw-tooth wave, and generating a PWM signal that drives the DCxe2x80x94DC converter. In the invention, the error amplification circuit is configured by cascading together a first analog amplification circuit having a fixed operation point, and a second analog amplification circuit having a fixed operation point, and the first analog amplification circuit is a differential amplification circuit.
In the second aspect of the present invention, the second analog amplification circuit in the first aspect may be a PI (proportional integration) control circuit.
The third aspect of the present invention provides a PWM control circuit for a DCxe2x80x94DC converter comprising a detection circuit for detecting an output voltage from a DCxe2x80x94DC converter that converts one DC voltage into another DC voltage by turning on and off a semiconductor switch, an error amplification circuit for amplifying a difference between the detected voltage and the reference voltage, and a comparison circuit for comparing the output voltage from the error amplification circuit with a carrier signal with a triangular wave or saw-tooth wave, and generating a PWM signal that drives the DCxe2x80x94DC converter. In the invention, the error amplification circuit is configured by synthesizing a differential amplification circuit and a PI control circuit together, and has a fixed operation point.
In the fourth aspect of the invention, the drive power supply for the PWM control circuit in the first to third aspects may be a single positive power supply having a negative-side terminal connected to a ground potential. The comparison circuit or the DCxe2x80x94DC converter may be configured so that the output voltage from the DCxe2x80x94DC converter is at its minimum when the output voltage from the error amplification circuit equals to the amplitude lower-limit potential of the carrier signal. The operation point of the error amplification circuit may be set at a potential between the amplitude lower-limit potential of the carrier signal and the ground potential.
In the fifth aspect of the present invention, when one or more capacitors are connected to a passive element for determining the gain of the error amplification circuit in the fourth aspect, a switch is connected parallel to the capacitors, and the switch is turned on when the DCxe2x80x94DC converter and the PWM control circuit are stopped.